Why does my video editing playback slow down after a short amount of time?
Seems likely you just run out of the buffer. Because the video frame is actually quite HUGE to computer system, the buffer of the video will be constraint at certain amount for sure to ensure available RAM.

The default setting amount can be find in Setting > System > Memory &amp; Limit: Sequencer Cache Limit



You should tune this amount depend on your RAM size. Yeah, 1024 MB is quite small for general 3D modeling available computer which usually got 16 GB or higher RAM.